1. Field of the Invention
The invention is directed to an improved mass flow measuring device for measuring the mass of a flowing medium .
2. Description of the Prior Art
A mass flow measuring device with a measurement conduit is already known (German Patent Disclosure DE 197 35 891 A1), which conduit accommodates a measuring element that is bathed by the inflowing medium there. The flowing medium flows from an inlet conduit first into a deflection conduit, which has a larger flow cross section than the inlet conduit and has a right-angled corner, so that there is an abrupt flow transition in the form of a shoulder toward the inlet conduit. Adjoining that, the medium flows from the deflection conduit, deflected by the corner, along the peripheral face of the deflection conduit into an outlet conduit adjoining it transversely, and leaves the outlet conduit through an outlet opening so that it can mix again with the medium flowing past the device. An inlet conduit longitudinal axis and an outlet conduit longitudinal axis are inclined by a predetermined angle relative to the longitudinal axis of the line, so that the inlet conduit has a region that is shaded from a primary flow direction. The measuring element is disposed in the shaded region of the measurement conduit, to prevent soiling and resultant defects of the measuring element.
Dirt particles that enter the inlet conduit along with the flowing medium can destroy the measuring element, if the dirt particles collide with it. Especially if micromechanical components, of the kind described in German Patent Disclosure DE 43 38 891 A1, for instance, are used as measuring elements, the dirt particles can strike a relatively thin diaphragm and permanently damage it. This can lead to increased wear of the measuring element and premature failure. Dirt particles that contain oil or grease can also become deposited on the measuring element, and especially on its diaphragm; they can act as adhesion promoters for solid particles, such as dust, and can permanently soil the measuring element. The soiling interferes with the thermal coupling between the measuring element and the flowing medium, causing a shift in a measurement characteristic curve that necessarily leads to measurement errors and thus incorrect triggering of the fuel injection valves.
From German Patent Disclosure DE 196 23 334 A1, it is known that the inlet conduit of such a device has a rectangular cross section; two side faces toward the chiplike measuring element are embodied as extending obliquely, resulting in a narrowing of the inlet conduit in the flow direction of the medium in the inlet conduit. A top face of the inlet conduit extending transversely to the side faces, from which top face the measuring element protrudes, and a bottom face of the inlet conduit opposite the top face, extend plane or parallel, with a constant spacing from one another. A device equipped with this kind of inlet conduit is also known from SAE Paper 950433 (International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995, reprinted from: Electronic Engine Controls 1995 (SP-1082)). As can be seen from the sectional view in FIG. 7, top, on page 108 of this publication, the inlet conduit and the deflection/outlet conduit are essentially formed of two parts; a part hereinafter called the bottom part, together with the measuring element includes a side face, a top face, and a bottom face of the measurement conduit. Another part has only the second side face of the measurement conduit and thus forms a cap part. The bottom part and the cap part are made from plastic by plastic injection molding. The narrowing design of the side faces of the inlet conduit results in an increasing wall thickness in the flow direction.
In an internal combustion engine, opening and closing of the injection valves of the individual cylinders cause considerable fluctuations or pulsations in the flow, the severity of which depends on the intake frequency of the individual pistons and on the engine rpm. The flow pulsations propagate from the injection valves along the intake line to the measuring element in the inlet conduit and onward from there. The effect of the pulsations is that depending on their severity, because of thermal inertia and directional insensitivity of the measuring element, the measuring element produces a measurement result that can deviate considerably from the flow speed prevailing in the inlet conduit and from the resultant calculated intake air flow rate of the engine. The inlet conduit and the deflection/outlet conduits are adapted to one another in their dimensions such that when there is a pulsating flow in the intake line, the erroneous indication provided by the measuring element as a result of the flow fluctuations is minimal. Nevertheless, at high pulsation frequencies and a significant pulsation amplitude, flow and/or acoustical processes taking place in the deflection conduit can lead to an erroneous indication of the aspirated air flow rate. This erroneous indication arises especially because when there is a pulsating flow downstream of the measuring element at the shoulder between the outlet of the inlet conduit and the corner at the first portion of the deflection conduit, a pressure wave can occur, which is reflected from the peripheral face of the deflection conduit at the corner, so that feedback interferes with a measurement signal of the measuring element.
From German Patent Disclosure DE 197 41 031 A1, a measurement device with an inlet conduit is known, in which device, by the design of two walls of the inlet conduit, an acceleration of the flow in the inlet conduit can continue to be maintained; this acceleration is known to lead to a stabilization of the flow of the medium in the inlet conduit, especially at the inlet.
However, the known devices have at least two of the following disadvantages:
they do not offer adequate protection of the measuring element from dirt;
a flow around the sensor carrier and poor stabilization of the flow in the inlet conduit lead to scattering of the measurement signal;
narrowing of the inlet conduit in only one direction, or in other words two opposed side walls;
inadequate provisions, if any, for improved pulsation performance;
disadvantages in terms of production: the entire measuring device would have to be tilted for improved protection against dirt, with the resultant changes in the measurement stub into which the measurement device is inserted; and
because of the increasing wall thickness of the plastic, different cooling speeds occur along with accumulations of material, which can in particular cause sunken areas on the side faces of the measurement conduit and which, in planned mass production of the device, would cause more or less severe scattering of the attainable measurement precision of the devices.
The improved flow measuring device according to the invention has the advantage over the prior art that in a simple way, the measurement performance is improved by reducing systematic and static errors, such as pulsation of the flow, by reduced soiling, and by improved flow behavior of the medium specifically.
Characteristics of claims 2-7 and 21 have the advantage improved stabilization of the flow in the measurement conduit, improved protection from dirt particles, and improvement at in the pulsation behavior are acheived.
The sealing of the sensor carrier at the bypass cap, the narrowing, the streamlined embodiment of all four peripheral faces of the inlet conduit, and the generally S-shaped embodiment of the measurement conduit all stabilize the flow in the measurement conduit.
Because of the oblique front edges of the sensor carrier and because of transverse flow components resulting from the inclination of the inlet conduit at a tangent to the respective edge of the sensor carrier, liquid and solid contaminants are carried away during operation. The shaded region prevents further accumulation of dirt particles. A suitable embodiment of an edge of the bow of the measurement housing and of a side wall of the inlet opening contribute to reflecting dirt particles away from the inlet opening.
Erroneous indications that occur when pulsation frequencies are high are reduced by the provision that a protuberance is provided in a surrounding region of the outlet opening, and a peripheral face of a first portion of the deflection conduit is embodied as inclined toward the flow direction in the measurement conduit. A fluidic connection or communication with the outer flow in the intake line, provided in the deflection conduit and taking the form of an opening, reduces any residual interference with the pressure wave that may still exist in the deflection conduit.